Vertical Datums

Elevation uses a vertical datum, which is distinct from the horizontal datum.  We refer elevations to sea level, but in practice that is difficult to measure (especially on the continents).  We have already referred to the ellipsoid, the best fitting shape of the earth.  It is very close to the geoid, a gravitational equipotential surface.  This means that every location on the geoid has the same value of gravity, directed perpendicular to the surface, so there is no tendency for water to flow downhill because the water molecules don’t see a hill.  The geoid varies from the ellipsoid by 100 m, primarily due to mass imbalances in the mantle (“fossilized plate tectonics”), and the large changes occur over 1000s of  km.  Smaller scale changes reflect ocean ridges and trenches, but these are only about 10 m and occur over hundreds of km so the slopes are very small.  Even smaller scale geoid changes of  1 m reflect ocean currents, over 10's of km.  Satellite radar altimeters like Geosat and Topex/Poseidon have measured theses changes in the ocean geoid to an accuracy and precision level of a few cm.

Vertical datums come in three categories: those based on a form of Mean Sea Level (MSL), called orthometric datums, those based on tidally-derived surfaces of high or low water, called tidal datums, and three-dimensional (3-D) datums realized through space-based systems such as Global Positioning System (GPS) which are referenced to the ellipsoid.  The "ortho" in orthometric means correct, because we expect heights to be in terms of sea level even if we don't normally think about how hard it is to measure sea level.

Height Measurements

Fig.   Definitions of height (from an old NIMA web page)

 

 

Datum Height Example Creation Characteristics Datum transformation
National or continental Orthometric NAVD 1929, NGVD 1988 Traditional survey and leveling, using bench marks, and tied to tide station(s) Used for maps on land.   
Geodetic Geoid EGM2008, EGM 96 Created from Geoid model from satellite observations with radar altimeters    NGA provides grids with differences between WGS84 ellipsoid and EGM 2008 and EGM96, and works worldwide
Ellipsoidal Ellipsoidal WGS84 Derived from horizontal datum and satellite measurments Instruments such as GPS or ICESAT measure elevation relative to the ellipsoid. These can either be transformed to a geodetic or national datum, or left as ellipsoidal. NGA provides grids with differences bewteen WGS84 ellipsoid and EGM 2008 and EGM96, and works worldwide
Tidal Orthometric Local chart datum from nautical chart with tide gauge Created from tide record from 18 year Metonic cycle Applies to a single tide station with epoch, and updated roughly every 18 years. Difficult to extend from the tide gauge.  Not used for maps on land. NOAA Vdatum provides some capabilities for this in the US
Can be on MSL, MLLW, or potentially others

 

Topographic maps from the USGS generally have elevations referenced to an orthometric datum, either the North American Vertical Datum 1988 (NAVD 88) or to the older National Geodetic Vertical Datum 1929 (NGVD 29).  Nautical charts have depths referenced to different tidal datums, which can vary from chart to chart. In the United States, mean lower low water (MLLW) is the official NOAA chart datum. To support harbor and river navigation, bridge clearances are referenced to a mean high water (MHW), and not MLLW.   In both cases, the choice provides a margin of safety--the water will generally be deeper than depicted, and the bridge clearance will be more.

On land, leveling surveys connect ocean tide gauges and lead to a national or continental survey adjustment.  These are done about as often as the vertical datum adjustments, so that the United States has NGVD 1929 and NAVD 88 which correspond roughly with NAD27 and NAD83;  this will be updated in 2022.  The vertical datum defines a surface to which all surveys and maps refer.  Differences in the orthometric vertical datums are usually small (a meter or less), and only affect very precise work.  This can often be the case along the coast, where nautical charts introduce another complexity with tidal datums.  This is now becoming more important with the increasing availability of LIDAR topography and looking at the impact of rising sea level, where a meter difference is huge.

Nautical charts use as their reference level MLLW.  This is the average of the lower of the two daily low tides, and while it makes for safer boating (there should almost always be deeper water than indicated), it requires an adjustment to get to the vertical datum used on land.   A nautical chart’s MLLW uses a local tidal gauge and an 18 year tidal record to establish a tidal epoch; these epochs are revised much more frequently than the vertical datums on land.  This conversion must be done for local areas, and NOAA has a tool for the conversion at http://vdatum.noaa.gov/. It only works in the US, and requires large data files to make the conversions, particularly for the local tide datums which are only valid for the particular tide station.

When a GPS measures elevation, it most often measures the Ellipsoid height because the ellipsoid is the simplest of the surfaces to define and calculate. This can often be below 0.

Key Global Regional DEMs can either use orthometric or ellipsoid elevations for the 0 point.  If the conversion is from the WGS84 ellipsoid (which is the same as ETRF89 or NAD83), they can use grids of earth gravity and the geoid from NGA to perform a vertical datum shift which is required to evaluate DEMs on different vertical datums, or to place the elevations on the geoid (mean sea level) so negative elevations only occur in places like the Dead Sea or Death Valley.

ASPRS had a highlight article on vertical datum transformations in the November 2009 issue.  This article describes the implementation of vertical datum adjustments in a particular piece of commercial software (GeoCue), but notes that NOAA and the National Geodetic Survey want more software to deal with tidal datums.  This will be required to integrate bathymetric data sets and coastal LIDAR. 

 

Outside references


Last revision 11/10//2021